Processes for cleaning and drying microelectronic structures using liquid or supercritical carbon dioxide

ABSTRACT

A method of cleaning and removing water and entrained solutes during a manufacturing process from a microelectronic device such as a resist-coated semiconductor substrate, a MEM&#39;s device, or an optoelectronic device comprising the steps of: (a) providing a partially fabricated integrated circuit, MEM&#39;s device, or optoelectronic device having water and entrained solutes on the substrate; (b) providing a densified (e.g., liquid or supercritical) carbon dioxide drying composition, the drying composition comprising carbon dioxide and a drying adjunct, the drying adjunct selected from the group consisting of cosolvents, surfactants, and combinations thereof; (c) immersing the surface portion in the densified carbon dioxide drying composition; and then (d) removing the drying composition from the surface portion. Process parameters are controlled so that the drying composition is maintained as a homogeneous composition during the immersing step, the removing step, or both the immersing and removing step, without substantial deposition of the drying/cleaning adjunct or entrained solutes on the substrate.

RELATED APPLICATIONS

This application claims the benefit of Provisional Application Serial No. 60/269,026, filed Feb. 15, 2001, the disclosure of which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention concerns methods and apparatus for removing water and aqueous-borne solutes from substrates such as semiconductor substrates, MEM's, or optoelectronic devices with liquid or supercritical carbon dioxide

BACKGROUND OF THE INVENTION

Production of integrated circuits, microelectronic devices, and micro-electo mechanical devices, (MEM's) involve multiple processing steps many of which incorporate water as either a carrier of chemistry, or a media to facilitate the removal of process byproducts. The evolution of materials and processes has been lead by a drive toward smaller feature sizes and more complex microdevices. In some cases, the use of water in these evolving processes has resulted in challenges whereby deleterious effects of water and byproducts carried by water have been seen. The unique physical properties of dense carbon dioxide in a liquid or supercritical state are of particular interest in preventing certain of these pitfalls.

One such process where dense CO₂ is of practical application relates to prevention of surface tension or capillary force induced image collapse. This is of particular interest during the aqueous development of micro-lithographic images using photoresists. Photoresists are photosensitive films used for transfer of images to a substrate. A coating layer of a photoresist is formed on a substrate and the photoresist layer is then exposed, through a photomask or by other techniques, to a source of activating radiation. Exposure to activating radiation provides a photoinduced chemical transformation of the photoresist coating to thereby transfer the pattern of the photomask (or other pattern generator) to the photoresist coated substrate. Following exposure, the photoresist is developed to provide a relief image that permits selective processing of a substrate. See. e.g., U.S. Pat. No. 6,042,997.

A photoresist can be either positive-acting or negative-acting. For negative acting resists, the solubility of the exposed region is decreased such that it remains on the wafer during development while the non-exposed region is removed. For positive acting resists the solubility of the exposed region increases in the developer solution, so it is removed during the development step leaving the unexposed region unaffected. Positive and negative acting resist materials typically incorporate chemical functionality that undergoes a transformation upon exposure to UV light at a given wavelength. The transformation is often referred to as a “polarity switch” because polymer polarity increases or decreases are often the driving force for changes in the solubility of the polymer in the developing solution. This transformation is facilitated by the incorporation of photoacid generators (PAG's) or photobase generators (PGB's) into the resist compositions. The acid and base moieties are typically generated upon exposure to the appropriate source of radiation followed by heat. The developer solutions are typically aqueous, and are typically dried from the substrate before further processing.

Capillary forces present in the aqueous drying of imaged resist patterns can result in resist deformation and pattern collapse. This problem becomes particularly serious as lithography techniques move toward smaller image nodes with larger aspect ratios. Researchers have suggested that collapse problems associated with aqueous drying will affect the 130-nm technology node, and will become more prevalent in subsequent technologies as aspect ratios increase.

Researchers at both IBM and NTT have suggested that the use of carbon dioxide in supercritical resist drying (SRD) may reduce image collapse and film damage. See, e.g., H. Namatsu, J. Vac. Sci. Technol. B 18(6), 3308-3312 (2000); D. Goldfarb et al., J. Vac. Sci. Technol B. 18(6) 3313-3317 (2000). However, while the absence of surface tension and the accessible critical temperature and pressure of CO₂ have been touted as positives factors for this drying approach, the relatively low solubility of water in the supercritical phase has also been described as a challenge that may necessitate the use of chemical adjuncts to increase the transport capacity of the fluid. Researchers at IBM and NTT have demonstrated the use of certain surfactants in supercritical fluid-aided drying. However, the surfactant is described as being incorporated into a hexane pre-rinse in “indirect SRD” See, e.g., Goldfarb et al., supra, or only particular surfactants have been incorporated into the carbon dioxide in “direct SRD”. In both the direct and indirect drying methods the choice of surfactants and co-solvents is limited by what is described as compatibility issues leading to resist damage. Accordingly, there remains a need for new approaches to SRD using carbon dioxide.

Another problem with drying of surfaces on microelectronic substrates (e.g. photoresist coated semiconductor wafers, MEMS, opto-electronic devices, photonic devices, flat panel displays, etc) is the complete removal of aqueous processing, cleaning or rinsing solutions without leaving a residue, commonly referred to as a drying watermark. These watermarks result from the concentration of solutes in the aqueous processing, cleaning, or drying fluid, as said fluid is dried. In many microelectronic, optical, micro-optical, or MEMS structures this watermark can negatively impact the manufacturing yield or ultimate performance of the device. There needs to be an effective method to remove (clean) water-based fluids from surfaces that eliminates the concentration and ultimate deposition of entrained solutes—eliminating watermarks.

One such challenge comes in the manufacturing of MEM's devices. Wet-processing steps generally culminate with a rinse and dry step. Evaporative drying causes water with low levels of solutes that is pooled on the surface and in various micro-features to concentrate in locations that maximize the surface area of the pool. As a result, these drying steps can lead to the concentration of once dissolved solutes in close proximity to or on motive parts. The deposited materials which can be organic or inorganic in nature contribute to stiction, the locking of the motive part such that it cannot be actuated. “Release stiction” as it is termed during the manufacturing step results, is believed to be derived from adhesive and Van der Waals forces and friction. The forces generated by this phenomenon can completely incapacitate motive parts on MEM's devices.

To combat stiction manufacturers of MEM's devices use solvents such as small chain alcohols that reduce surface tension during the rinse step and facilitate a more even drying process. However, these steps alone have not eliminated the occurrence of stiction. Supercritical CO₂ has been proposed for drying microstructures, (see Gregory T. Mulhern “Supercritical Carbon Dioxide Drying of Micro Structures”) where surface tension forces can cause damage. Researchers at Texas Instruments Inc. among others (see, e.g., U.S. Pat. No. 6,024,801) have demonstrated that supercritical CO₂ can be used to clean organic and inorganic contaminants from MEM's devices prior to a pacification step, thus limiting stiction.

These technologies utilizing supercritical CO₂ do not limit stiction by combination of drying and cleaning where water and solutes are removed simultaneously so to avoid the concentration of water and solutes at specific site. Technologies are needed that can prevent release stiction through an integrated process of drying, cleaning, and surface pacification.

Other examples of drying and cleaning challenges related to aqueous wet-processing steps come in the formation of deep vias for interlayer metalization in the production of integrated circuits. These vias, formed by methods known to those familiar with the art, typically have large critical aspect ratios creating geometries that can be difficult to clean residues from. Furthermore, wet-processing steps and rinses with traditional fluids such as water leave once dissolved solutes behind upon evaporative drying. These solutes deposited at the bottom of the vias can inhibit conduction upon metalization lowering functional yields.

Technologies are needed that remove water (dry) and dissolved solutes (clean) from vias after wet processing steps, thus reducing yield losses.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a method of cleaning a microelectronic device, comprising the steps of: providing a substrate having a surface portion to be cleaned, providing a densified carbon dioxide cleaning composition, the composition comprising carbon dioxide and a cleaning adjunct, the cleaning adjunct selected from the group consisting of cosolvents, surfactants, and combinations thereof; immersing the surface portion in the densified carbon dioxide composition; and then removing said cleaning composition from the surface portion; while maintaining the cleaning composition as a homogeneous composition during at least one of said immersing step and said removing step. Examples of devices that may be cleaned by the present invention include, but are not limited to, microelectromechanical devices (MEMs), optoelectronic devices, and resist-coated substrates.

In a particular embodiment the present invention provides a method for removing water and entrained solutes from a microelectronic device such as resist-coated substrate (such as a semiconductor substrate), a MEMs device, or an optoelectronic device is disclosed herein. In such a method, the cleaning/removal of the water may also be referred to as “drying” of the water from the device. In general, the method comprises the steps of: (a) providing a substrate having a an imaged or patterned feature such as a resist coated silicon wafer and having water on the resist coating; (b) providing a densified (e.g., liquid or supercritical) carbon dioxide drying composition, the drying composition comprising carbon dioxide and a drying adjunct, the drying adjunct selected from the group consisting of cosolvents, surfactants, and combinations thereof; (c) immersing the surface portion in the densified carbon dioxide drying composition; and then (d) removing the drying composition from the surface portion.

In the present invention, process parameters are controlled so that the drying and cleaning composition is maintained as a homogeneous composition during the immersing step, the removing step, or both the immersing and removing step, without substantial deposition of the drying adjunct or the aqueous entrained solutes on the resist coating, the patterned feature, or the mechanical, electrical, or optical components of the device or circuit.

The resist typically comprises a polymeric material, and may be a positive-acting resist or a negative-acting resist. The resist may be patterned or unpatterned, developed or undeveloped at the time the drying process is carried out.

The present invention is explained in greater detail in the drawings and specification set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a substrate having a patterned resist layer formed thereon, with water present in various locations thereon.

FIG. 2 illustrates an apparatus for carrying out the methods of the present invention.

FIG. 3 depicts a phase diagram of predominantly CO₂ system representing the plausability of a transition from a predominantly CO₂ supercritical mixture to a gas avoiding a liquid phase.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Any suitable resist composition can be used to carry out the present invention, including but not limited to those described in U.S. Pat. Nos. 6,042,997; 5,866,304; 5,492,793; 5,443,690; 5,071,730; 4,980,264; and 4,491,628. Applicants specifically intend that the disclosures of all United States patent references that are cited herein be incorporated herein by reference in their entirety.

The resist compositions may be applied to the substrate as a liquid compositions in accordance with generally known procedures, such as by spinning, dipping, roller coating or other conventional coating technique. When spin coating, the solids content of the coating solution can be adjusted to provide a desired film thickness based upon the specific spinning equipment utilized, the viscosity of the solution, the speed of the spinner and the amount of time allowed for spinning.

The resist compositions are suitably applied to substrates conventionally used in processes involving coating with photoresists. For example, the composition may be applied over silicon wafers (that may include one or more layers thereon such as silicon dioxide, silicon nitride, polysiloxand and/or metal, etc.) for the production of microprocessors and other integrated circuit components. Aluminum-aluminum oxide, gallium arsenide, ceramic, quartz or copper substrates also may be employed. Substrates used for liquid crystal display and other flat panel display applications are also suitably employed, e.g. glass substrates, indium tin oxide coated substrates and the like.

Following coating of the photoresist onto a surface, it is dried by heating to remove the solvent until preferably the photoresist coating is tack free. Alternatively it may be dried by the procedures described herein. Thereafter, it is imaged in a conventional manner. The exposure is sufficient to effectively activate the photoactive component of the photoresist system to produce a patterned image in the resist coating layer.

Following exposure, the film layer of the composition may be baked. Thereafter, the film is developed by contacting the film resist layer to any suitable developer solution (the choice of which will depend in part upon the particular choice of resist material). For example, the developer may be a polar developer, for example an aqueous based developer such as an inorganic alkali exemplified by sodium hydroxide, potassium hydroxide, sodium carbonate, sodium bicarbonate, sodium silicate, sodium metasilicate; quaternary ammonium hydroxide solutions such as a tetra-alkyl ammonium hydroxide solution; various amine solutions such as ethyl amine, n-propyl amine, diethyl amine, di-n-propyl amine, triethyl amine, or methyldiethyl amine; alcohol amines such as diethanol amine or triethanol amine; cyclic amines such as pyrrole, pyridine, etc. In general, development is in accordance with art recognized procedures. After development the resist is optionally rinsed (for example with an aqueous rinse) and is then dried, preferably by the drying procedures described herein.

Following development of the photoresist coating over the substrate, the developed substrate may be selectively processed on those areas bared of resist, for example by chemically etching or depositing on substrate areas bared of resist in accordance with procedures known in the art. For the manufacture of microelectronic substrates, e.g., the manufacture of silicon dioxide wafers, suitable etchants include a gas etchant, e.g. a chlorine or fluorine-based etchant such a CF₄ or CF₄/CHF₃ etchant applied as a plasma stream, in accordance with known techniques.

Carbon-dioxide cleaning drying compositions used to carry out the present invention typically comprise:

(a) carbon dioxide to balance, typically at least 20, 30, 40, 50 or 60 percent;

(b) from 0, 0.01, 0.1, 0.5, 1 or 2 percent to 5 or 10 percent or more of surfactant;

(c) from 0, 0.01, 0.1, 1 or 2 to 30, 40 or 50 percent or more of an organic co-solvent; and

(d) optionally, from 0, 0.01, or 0.1 to 2 or 5 percent water.

Preferably at least one of the surfactant and/or the co-solvent is included (e.g., by at least 0.01 percent) in the cleaning/drying composition, and optionally both a surfactant and a co-solvent may be included in the composition. Water may or may not be included in the composition, depending upon the particular cleaning application and the nature of the substrate. Percentages herein are expressed as percentages by weight unless otherwise indicated.

The cleaning/drying composition may be provided as a liquid or supercritical fluid, including cryogenic liquids. Liquid and supercritical carbon dioxide are herein together referred to as “densified” carbon dioxide in accordance with established usage.

The organic co-solvent may be one compound or a mixture of two or more ingredients The organic co-solvent may be or comprise an alcohol (including diols, triols, etc.), ether, amine, ketone, carbonate, or alkanes, or hydrocarbon (aliphatic or aromatic) The organic co-solvent may be a mixture of compounds, such as mixtures of alkanes as given above, or mixtures of one or more alkanes in combination with additional compounds such as one or more alcohols as described above. (e.g., from 0 or 0.1 to 5% of a C1 to C15 alcohol (including diols, triols, etc.)). Any surfactant can be used to carry out the present invention, including both surfactants that contain a CO₂-philic group (such as described in PCT Application WO96/27704) linked to a CO₂-phobic group (e.g., a lipophilic group) and surfactants that do not contain a CO₂-philic group (i.e., surfactants that comprise a hydrophilic group linked to a hydrophobic (typically lipophilic) group). A single surfactant may be used, or a combination of surfactants may be used. Numerous surfactants are known to those skilled in the art. See, e.g., McCutcheon's Volume 1: Emulsifiers & Detergents (1995 North American Edition) (MC Publishing Co., 175 Rock Road, Glen Rock, N.J. 07452). Examples of the major surfactant types that can be used to carry out the present invention include the: alcohols, alkanolamides, alkanolamines, alkylaryl sulfonates, alkylaryl sulfonic acids, alkylbenzenes, amine acetates, amine oxides, amines, sulfonated amines and amides, betaine derivatives, block polymers, carboxylated alcohol or alkylphenol ethoxylates, carboxylic acids and fatty acids, a diphenyl sulfonate derivatives, ethoxylated alcohols, ethoxylated alkylphenols, ethoxylated amines and/or amides, ethoxylated fatty acids, ethoxylated fatty esters and oils, fatty esters, fluorocarbon-based surfactants, glycerol esters, glycol esters, hetocyclic-type products, imidazolines and imidazoline derivatives, isethionates, lanolin-based derivatives, lecithin and lecithin derivatives, lignin and lignin derivatives, maleic or succinic anhydrides, methyl esters, monoglycerides and derivatives, olefin sulfonates, phosphate esters, phosphorous organic derivatives, polyethylene glycols, polymeric (polysaccharides, acrylic acid, and acrylamide) surfactants, propoxylated and ethoxylated fatty acids alcohols or alkyl phenols, protein-based surfactants, quaternary surfactants, sarcosine derivatives, silicone-based surfactants, soaps, sorbitan derivatives, sucrose and glucose esters and derivatives, sulfates and sulfonates of oils and fatty acids, sulfates and sulfonates ethoxylated alkylphenols, sulfates of alcohols, sulfates of ethoxylated alcohols, sulfates of fatty esters, sulfonates of benzene, cumene, toluene and xylene, sulfonates of condensed naphthalenes, sulfonates of dodecyl and tridecylbenzenes, sulfonates of naphthalene and alkyl naphthalene, sulfonates of petroleum, sulfosuccinamates, sulfosuccinates and derivatives, taurates, thio and mercapto derivatives, tridecyl and dodecyl benzene sulfonic acids, etc.

FIG. 1 illustrates a resist-coated substrate article 10 to be dried by the method of the present invention. The article comprises a substrate 11, which may comprise silicon or any other suitable material as described above, and which may itself comprise one or more layers, having a resist coating 12 formed thereon. Water droplets 14, 15, to be removed by drying, are on the top surface and in a trench formed in the resist coating.

FIG. 2 schematically illustrates an apparatus for carrying out the method of the invention. The apparatus comprises an enclosed drying vessel 21, suitable for containing liquid or supercritical carbon dioxide, in which vessel the coated substrate 10 (or other microelectronic device to be cleaned) is positioned on a suitable support 27. The drying vessel may include a door, a stirring device or other means of agitation, a view window, a compressor connected to the drying vessel to increase or decrease the pressure therein, a heat exchanger, heater or cooler connected to the drying vessel to increase or decrease the temperature of the contents thereof, etc.

A carbon dioxide cleaning/drying composition supply 22 is connected to the drying vessel by appropriate piping. The cleaning/drying composition supply 22 may itself comprise one or more storage vessels, pumps, valves, piping for mixing the drying adjunct into the carbon dioxide, etc. The vessel may be filled with the cleaning/drying composition to a level 28 above the article to be cleaned 10.

Depending upon the particular technique or combination of techniques being employed to control the processing conditions, the system includes a supply of a second gas, second material, and/or additional carbon dioxide 24 connected to the drying vessel 21.

If desired, a developer solution supply 25 may be connected to the vessel so that both development and drying of the substrate may be carried out in the same vessel 21.

A draining system 26 is preferably connected to the vessel 21 for draining whatever composition is contained therein. The draining system may itself comprise appropriate pumps, valves, compressors and the like (some of which components may be serve multiple functions in conjunction with supply elements described above), may include a still for distilling and optionally recycling ingredients such as carbon dioxide, and may include suitable piping, valves, etc. for recycling various compositions or constituents thereof to supply elements for re-use. For example, used drying composition may be distilled to allow carbon dioxide to be recycled and re-used as part of the drying composition, or to the source of additional carbon dioxide supply.

As noted above, the method of the invention comprises the steps of:

(a) providing a substrate having a an imaged or patterned feature such as a resist coated silicon wafer and having water on the resist coating;

(b) providing a densified (e.g., liquid or supercritical) carbon dioxide drying composition, the drying composition comprising carbon dioxide and a drying adjunct, the drying adjunct selected from the group consisting of cosolvents, surfactants, and combinations thereof;

(c) immersing the surface portion in the densified carbon dioxide drying composition; and then

(d) removing the drying composition from the surface portion. The process parameters are controlled so that the drying composition is maintained as a homogeneous composition during the immersing step, the removing step, or both the immersing and removing step, without substantial deposition of the drying adjunct on the resist coating.

Preferably, the providing step is carried out by mixing the carbon dioxide with the adjunct to produce a homogeneous solution, and then the immersing step is carried out while maintaining the drying composition as a homogeneous solution. Such mixing can be carried out in the drying composition supply 22 by any suitable means, such as stirring, injection under pressure, etc.

The removing step is preferably carried out while maintaining the drying composition as a homogeneous solution. In general, this is achieved by inhibiting the boiling of the drying composition as it is drained from the drying vessel. When draining liquid CO₂ from a vessel the liquid reaches a state where it is at equilibrium with CO₂ vapor, termed saturated vapor pressure. To maintain saturation, as liquid is removed from the vessel by venting or pumping preferably from the bottom of the vessel, the liquid phase boils to generate vapor for the increasing volume of the vapor phase. This boiling which may be nucleated at liquid/gas, and liquid/solid interfaces causes adjuncts with lower vapor pressure than CO₂ including, co-solvents and surfactants, and solute contaminants to concentrate at interfaces. Concentrated adjuncts, deposited contaminants and interfacial stresses created by boiling at liquid/solid interfaces can be damaging to resist features, MEM's, or other patterned microdevices. In the case of imaged and developed resists, feature sizes less 130-nm with aspect ratios greater than 3 are particularly susceptible to damage. Process controls to prevent such damage are as follows.

For example, when the drying composition is a liquid drying composition, the removing step may be carried out by pressurizing the enclosed chamber with a second compressed gas (e.g., helium, nitrogen, air, mixtures thereof) from supply 24 by an amount sufficient to inhibit boiling of the drying composition during the draining step. The second gas is preferably one that is substantially immiscible in the drying composition possessing a saturated vapor pressure that is higher than CO₂. The second gas may be used to itself force the drying composition from the vessel, or the drying composition may be pumped or otherwise drained from the vessel while the second gas maintains an over-pressurization at the gas-liquid interface formed in the wash vessel during draining thereof.

Alternatively, if the drying composition is in the liquid phase, the draining step can be accomplished without boiling by liquid-gas equilibration with a secondary chamber or storage vessel. In this scenario, drying chamber 21 is connected to storage vessel 31 by gas-side line 32 (top), and liquid-side line 33. Each line contains a valve 34, 35 to separate or isolate vessels 21 and 31 from one another. During the draining step, storage vessel 31 contains a liquid CO₂ composition at a saturated pressure equal to or in excess of the saturated vapor pressure in the cleaning/drying vessel 21. Draining may be accomplished by first opening the gas-side connection 32 between vessels 21 and 31, and then opening the liquid-side connection 33. Liquid flows from cleaning vessel 21 to storage vessel 31 by gravity, if 21 is located sufficiently above 31, and/or by pumping. Liquid transfer described above avoids boiling thereby avoiding potential damage to resist features or other device features.

When the drying composition is a supercritical drying composition there will not be a gas-liquid interface. In this case, the removing step may be carried out by first adding a second material (e.g., a cosolvent as described above or a secondary gas) to the supercritical drying composition so that it is converted to a liquid drying composition, which can then be removed from the vessel as described above. If a secondary gas is used to cause the supercritical fluid phase to change to a liquid, the gas should be chosen from those having a saturated vapor pressure that is higher than that of CO₂ and/or a critical pressure and temperature higher than that of CO₂. Exemplary gases include but are not limited to: nitrogen, argon, helium, oxygen, and mixtures thereof.

Alternatively, when the drying composition is in the supercritical state, the adjunct containing fluid can be sufficiently diluted prior to the draining step by simultaneous addition of pure supercritical CO₂ and removal of adjunct-containing supercritical CO₂. After sufficient fluid turnover is accomplished and adjunct concentration is effectively minimized, the supercritical fluid is vented from the drying vessel by maintaining the fluid in the supercritical state until a transition is made directly to the gas state thus avoiding the liquid state. This is accomplished during the draining/venting step by maintaining the fluid temperature above the critical temperature of the mixture (Tc) until the pressure in the vessel is below the critical pressure of the mixture (Pc). FIG. 3 depicts a phase diagram of predominantly CO₂ system representing the plausibility of a transition from a predominantly CO₂ supercritical mixture to a gas avoiding a liquid phase. Because the expansion of the supercritical fluid and subsequent expansion of the remaining gas is an endothermic process, heat may need to be added to the system to maintain the temperature of the fluid or gas above the critical temperature thus avoiding condensation of the supercritical fluid or gas to a liquid or solid. By effecting a direct transition from the supercritical phase to the gas phase, liquid boiling is avoided thereby avoiding the interfacial stress caused by a retracting liquid meniscus at the liquid/solid interface, and unwanted deposition of solutes onto and in microstructures.

In another embodiment, the removing step is carried out by diluting the drying composition with additional carbon dioxide from supply 24, during which dilution the drying composition is removed from the vessel by draining system 23. Since larger quantities of carbon dioxide are required for such a technique, the use of a still to distill drained carbon dioxide, along with appropriate piping and valving for returning the carbon dioxide to supply 22 or supply 24 for subsequent re-use, is preferred.

In still another embodiment, a secondary gas is used, at a pressure range above the saturation point of CO₂ gas, to displace liquid and gaseous CO₂ in the drying chamber leaving a predominance of the secondary gas in the vapor phase. The secondary gas, possessing a lower heat of compression, can be vented from the chamber to ambient pressure with less heat loss to the system. Also represented by a smaller Joule-Thomson coefficient, (μ), the expansion of the gas from high pressure to atmospheric conditions results in less change in temperature at or in close proximity to the substrate. (μ_(CO2)>μ_(X), where X=secondary gas).

μ=(dT/dP)_(H)

In this embodiment, the secondary gas is useful in avoiding thermal shock when rapid cycling of pressure is desired for high throughput. Substrates such as silicon wafers can crack or become damage when significant temperature gradients exist in that substrate. Cooling of chambers and vessels from gaseous expansion can also add valuable processing time and require substantial heat input for temperature regulation. The use of a secondary gas can minimize heat loss and heat inputs, potentially reducing cycle time and energy requirements.

The present invention is explained in greater detail in the following non-limiting Examples.

Comparative Example A Treatment of a Coated Wafer with Liquid Carbon Dioxide

A CO₂-miscible, hydrophilic solvent, such as isopropanol (IPA), was added to a high-pressure vessel that contained a piece of a poly(hydroxystyrene) (PHS) coated silicon wafer. Liquid CO₂ was added to the high-pressure vessel. As the liquid CO₂/IPA (2% IPA by volume) mixture meniscus level rose above the surface of the wafer, damage to the wafer was observed. After the system was mixed for 15 minutes, the liquid CO₂/IPA mixture was drained from the bottom of the high-pressure vessel. More damage to the wafer was observed as the IPA boiled at the liquid/gas/wafer interface.

EXAMPLE 1 Treatment of a Coated Wafer with Liquid Carbon Dioxide

Liquid CO₂ was added to a high-pressure vessel that contained a piece of a PHS coated silicon wafer until the wafer was completely submerged in liquid CO₂. A mixture that contained liquid CO₂ and IPA, 2% IPA by volume, (alternatively any CO₂-miscible, hydrophilic solvent, or any hydrophilic/CO₂-philic surfactant) was added to the high-pressure vessel that contained the piece of PHS coated silicon wafer submerged in liquid CO₂. No damage to the wafer was observed. The system was mixed for 15 minutes. There was still no damage to the wafer. A secondary gas (helium or nitrogen) was added to the top of the high-pressure vessel. The liquid CO₂/IPA mixture was drained under the pressure of the secondary gas to prevent oiling at the liquid/gas/wafer interface. There was no damage to the wafer after the system was drained with a secondary gas. The system was rinsed with pure liquid CO₂ and was then drained as mentioned above. There was no damage to the wafer.

EXAMPLE 2 Treatment of a Coated Wafer with Liquid Carbon Dioxide

Liquid CO₂ at its saturated vapor pressure was added to a high-pressure vessel that contained a piece of a PHS coated silicon wafer until the wafer was completely submerged in liquid CO₂. A mixture that contained liquid CO₂ and IPA, 2% IPA by volume, (alternatively any CO₂-miscible, hydrophilic solvent, or hydrophilic/CO₂-philic surfactant) was added to the high-pressure vessel that contained the piece of PHS coated silicon wafer submerged in liquid CO₂. No damage to the wafer was observed. The liquid CO₂ mixture was drained from the high-pressure vessel to another high-pressure vessel containing predominantly liquid CO₂ at saturated vapor pressure by first opening a valve connecting the vapor side of both vessels then by opening a valve connecting the liquid side of both vessels. The liquid was drained by force of gravity as the first vessel was positioned substantially above the second to allow for complete drainage. No damage was observed. Pure liquid CO₂ was added to the vessel containing the wafer segment as a rinse, and that liquid was subsequently drained in the manor described above. Again, no damage was observed.

EXAMPLE 3 Treatment of a Coated Wafer with Liquid and Supercritical CO₂

Liquid CO₂ was added to a high-pressure vessel that contained a piece of a PHS coated silicon wafer until the wafer was completely submerged in liquid CO₂. A mixture that contained liquid CO₂ and IPA, 2% IPA by volume, (alternatively any CO₂-miscible, hydrophilic solvent or surfactant that increased the carry capacity of CO₂ for water) was added to the high-pressure vessel that contained the piece of PHS coated silicon wafer submerged in liquid CO₂. No damage to the wafer was observed. After a period of time sufficient to remove the substantial majority of the water from the surface of the wafer, the liquid mixture was diluted with pure liquid CO₂ to effect approximately 5 liquid turnovers in the drying chamber. Heat was then added to the liquid CO₂ causing a transition to the supercritical phase. The chamber containing the wafer is then drained and vented by maintaining the temperature of fluid and gas above the critical temperature of CO₂, thus avoiding the liquid phase. The wafer was removed from the chamber with no damage.

EXAMPLE 4 Treatment of a Coated Wafer with Supercritical Carbon Dioxide

Supercritical CO₂ was added to a high-pressure vessel that contained a piece of a PHS coated silicon wafer. A mixture that contained supercritical CO₂ and IPA, 2% IPA by volume, (alternatively any CO₂-miscible, hydrophilic solvent or surfactant that increased the carry capacity of CO₂ for water) was added to the high-pressure vessel that contained the piece of PHS coated silicon wafer and supercritical CO₂. No damage to the wafer was observed. The system was mixed for 15 minutes. There was still no damage to the wafer. A secondary gas (helium or nitrogen) was added to the top of the high-pressure vessel until the system became subcritical and a liquid meniscus was formed. The liquid CO₂/IPA mixture was drained under the pressure of the secondary gas to prevent boiling at the liquid/gas/wafer interface. There was no damage to the wafer after the system was drained with a secondary gas. The system was rinsed with pure liquid CO₂ and was then drained as mentioned above. There was no damage to the wafer.

Comparative Example B Solvation of Water from a Coated Wafer with Liquid Carbon Dioxide

A droplet of water was dripped on top of a piece of a PHS coated silicon wafer. The wafer that contained the water droplet was placed in the high-pressure view cell. Pure liquid CO₂ was added to the high-pressure vessel. The system was mixed for 15 minutes. The liquid CO₂ did not solvate the entire droplet of water as determined visually through a sapphire window on the view cell.

EXAMPLE 5 Solvation of Water from a Coated Wafer with Liquid Carbon Dioxide and Cosolvent

A droplet of water was dripped on top of a piece of a PHS coated silicon wafer. The wafer that contained the water droplet was placed in the high-pressure view cell. Liquid CO₂ was added to a high-pressure vessel that contained a piece of a PHS coated silicon wafer until the wafer was completely submerged in liquid CO₂. A mixture that contained liquid CO₂ and IPA, 2% IPA by volume, (alternatively any CO₂-miscible, hydrophilic solvent) was added to the high-pressure vessel that contained the piece of PHS coated silicon wafer submerged in liquid CO₂. No damage to the wafer was observed. The system was mixed for 15 minutes. The water droplet was completely solvated. There was still no damage to the wafer. A secondary gas (helium or nitrogen) was added to the top of the high-pressure vessel. The liquid CO₂/IPA mixture was drained under the pressure of the secondary gas to prevent boiling at the liquid/gas/wafer interface. There was no damage to the wafer after the system was drained with a secondary gas. The system was rinsed with pure liquid CO₂ and was then drained as mentioned above. There was no damage to the wafer.

EXAMPLE 6 Solvation of Water from a Coated Wafer with Liquid and Supercritical Carbon Dioxide and Cosolvent

A whole 5″ PHS coated wafer wetted with water, as it would be in an aqueous post-development process, was placed in the prototype drying chamber. The chamber was filled with liquid carbon dioxide. The prototype system contained a second high pressure vessel, containing liquid CO₂ plus 2% IPA by volume, (alternatively any CO₂-miscible, hydrophilic solvent or surfactant that increased the carry capacity of CO₂ for water) The mixed liquid CO₂/IPA was added to the drying chamber from the second high-pressure vessel using a pump. The system was mixed for 15 minutes. The liquid CO₂/IPA mixture was flushed with 5 liquid turnovers of pure liquid CO₂ so that the concentration of IPA dropped to a fraction of its previous concentration. There was no meniscus formation during the CO₂ flush. After the CO₂ flush, the liquid CO₂ was heated to 35° C. transitioning the fluid to a supercritical phase. The supercritical CO₂ was then drained/vented from the vessel as heat was added to maintain the fluid, and subsequently the gas, above the critical temperature of CO₂. When the chamber was completely vented the wafer was removed dry and undamaged.

EXAMPLE 7 Drying of Water from an Imaged and Aqueously Developed Resist-Coated Wafer Using CO₂ and Chemical Adjuncts

A 5-inch silicon wafer coated with a PHS photoresist and a PAG was imaged, developed using 0.238 normal tetramethyl ammonium hydroxide, and rinsed with deionized water. The wet wafer was then transferred to a high-pressure drying chamber, where liquid CO₂ at saturated vapor pressure was added in a small amount. Additional liquid CO₂ at saturated vapor pressure premixed with a hydrophilic/CO₂-philic surfactant was added to and circulated through the chamber to displace and remove the water from the surface of the wafer and features of the resist pattern. After a short period of time the liquid was drained to a secondary storage vessel containing a small amount of liquid CO₂ first by allowing a vapor-side communication between the two vessels then by opening a valve connecting the bottom of the drying vessel with the bottom of the second storage vessel. The second storage vessel was position sufficiently below the drying chamber that the majority of the liquid drained from the drying chamber. The drying chamber was then filled with pure liquid CO₂ as a rinse followed by draining as described above. This was repeated to insure that the concentration of the adjunct was effectively zero. The small amount of remaining liquid CO₂ in the drying chamber was heated to above its critical point, 35° C., and the CO₂ was vented while maintaining the fluid/gas temperature above the critical temperature thus avoiding the formation of a liquid meniscus. The imaged, developed, and dried wafer was then removed from the chamber, stored in the absence of light and moisture, and then analyzed using a scanning electron microscope. The micrograph showed that the developed features, demonstrating line/ space patterns of less than 120-nm, were consistent structurally unaffected by the CO₂ drying process.

EXAMPLE 8 MEM's Water and Contaminant Removal

During the manufacturing of a MEM's device containing a series of electrostatic actuators, a sacrificial oxide layer is removed using aqueous hydrofluoric acid, exposing a series of pivoting plates parallel to the substrate surface. After a sequential rinse step, the device is transferred to a high-pressure CO₂-based drying chamber, where a liquid CO₂ mixture is added at saturated vapor pressure. The liquid CO₂ contains a CO₂-philic/hydrophilic surfactant that is premixed with the CO₂ to ensure a homogeneous composition. After a period of circulation, pure liquid CO₂ is fed into the chamber as liquid CO₂, surfactant, water and entrained solutes are removed from the vessel at constant pressure. The liquid CO₂ remaining in the chamber is then heated to above its critical temperature converting the fluid into the supercritical state. The supercritical fluid in the processing chamber is then vented into a storage tank serving to ensure that the temperature of the fluid/gas mixture stays above the critical temperature of CO₂. This serves to ensure that the liquid state, a liquid meniscus, and associated surface tension are avoided during the draining/venting step. An SEM analysis of the MEM's device shows that the pivoting plates are all substantially parallel to the substrate surface with no evidence of release stiction. The foregoing is illustrative of the present invention, and is not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein. 

That which is claimed is:
 1. A method of cleaning a microelectronic device, comprising the steps of: providing a substrate having a surface portion to be cleaned, providing a densified carbon dioxide cleaning composition, said cleaning composition comprising carbon dioxide and a cleaning adjunct, said cleaning adjunct selected from the group consisting of cosolvents, surfactants, and combinations thereof; immersing said surface portion in said densified carbon dioxide composition; and then removing said cleaning composition from said surface portion; and homogenizing said cleaning composition during at least said removing step.
 2. The method according to claim 1, wherein said microelectronic device comprises a microelectromechanical device.
 3. The method according to claim 1, wherein said microelectronic device comprises an optoelectronic device.
 4. The method according to claim 1, wherein said microelectronic device comprises a resist-coated substrate.
 5. The method according to claim 1, wherein said carbon dioxide is supercritical carbon dioxide.
 6. The method according to claim 1, wherein said cleaning adjunct comprises a co-solvent and said cleaning composition is a homogenous composition.
 7. The method according to claim 6, wherein said co-solvent comprises an alkane, an alcohol, or a combination thereof.
 8. The method according to claim 1, wherein said cleaning adjunct comprises a surfactant and said cleaning composition is a homogenous composition.
 9. The method according to claim 8, wherein said surfactant contains a CO₂-philic group.
 10. The method according to claim 8, wherein said surfactant does not contain a CO₂-philic group.
 11. The method according to claim 1, wherein said providing step is carried out by mixing said carbon dioxide with said adjunct to produce a homogeneous solution.
 12. The method according to claim 11, wherein said immersing step is carried out while maintaining said cleaning composition as a homogeneous solution.
 13. The method according to claim 1, wherein said cleaning composition is a liquid cleaning composition, wherein said immersing and removing steps are carried out in an enclosed chamber, and wherein said homogenizing step is carried out by pressurizing said enclosed chamber with a second compressed gas by an amount sufficient to inhibit boiling of said cleaning composition.
 14. The method according to claim 13, wherein said second compressed gas is selected from the group consisting of helium, nitrogen, and air.
 15. A method according to claim 13, wherein said removing step is carried out by displacing liquid and gaseous CO₂ with a second compressed gas with a lower heat of compression than CO₂, thereby leaving a majority of said second gas in the vapor phase and serving to prevent thermal shock upon subsequent venting thereof.
 16. The method according to claim 1, wherein said cleaning composition is a supercritical cleaning composition, wherein said immersing and removing steps are carried out in an enclosed chamber, and wherein said homogenizing step is carried out by adding a second material to said supercritical cleaning composition so that it is converted to a homogenous liguid cleaning composition.
 17. The method according to claim 1, wherein said homogenizing step is carried out by diluting said cleaning composition with additional carbon dioxide.
 18. A method according to claim 1, wherein said cleaning step is initiated with said cleaning composition in a liquid state, and after a period of time said homogenizing step is carried out by diluting said cleaning composition with pure liquid CO₂ and then heating said cleaning composition to produce a supercritical fluid, after which said removing step is carried out while maintaining the temperature of the fluid and gas above the critical temperature of CO₂.
 19. A method according to claim 1, wherein said cleaning comprises removal of water from said microelectronic device.
 20. A method of removing water from a resist-coated substrate, comprising the steps of: providing a substrate having a resist coating formed on a surface portion thereof, and having water on said resist coating; providing a densified carbon dioxide drying composition, said drying composition comprising carbon dioxide and a drying adjunct, said drying adjunct selected from the group consisting of cosolvents, surfactants, and combinations thereof; immersing said surface portion in said densified carbon dioxide drying composition; and then removing said drying composition from said surface portion; and homogenizing said drying composition during at least said removing step.
 21. The method according to claim 20, wherein said substrate comprises a semiconductor substrate.
 22. The method according to claim 20, wherein said resist comprises a polymeric material.
 23. The method according to claim 20, wherein said resist is selected from the group consisting of positive-acting resists and negative-acting resists.
 24. The method according to claim 20, wherein said resist is a patterned resist.
 25. The method according to claim 20, wherein said carbon dioxide is liquid carbon dioxide.
 26. The method according to claim 20, wherein said carbon dioxide is supercritical carbon dioxide.
 27. The method according to claim 20, wherein said drying adjunct comprises a co-solvent.
 28. The method according to claim 27, wherein said co-solvent comprises an alkane, an alcohol, or a combination thereof.
 29. The method according to claim 20, wherein said drying adjunct comprises a surfactant.
 30. The method according to claim 29, wherein said surfactant contains a CO₂-philic group.
 31. The method according to claim 29, wherein said surfactant does not contain a CO₂-philic group.
 32. The method according to claim 20, wherein said providing step is carried out by mixing said carbon dioxide with said adjunct to produce a homogeneous solution.
 33. The method according to claim 32, wherein said immersing step is carried out while maintaining said drying composition as a homogeneous solution.
 34. The method according to claim 30, wherein said drying composition is a liquid drying composition, wherein said immersing and removing steps are carried out in an enclosed chamber, and wherein said homogenizing step is carried out by pressurizing said enclosed chamber with a second compressed gas by an amount sufficient to inhibit boiling of said drying composition.
 35. The method according to claim 34, wherein said second compressed gas is selected from the group consisting of helium, nitrogen, and air.
 36. A method according to claim 34, wherein said removing step is carried out by displacing liquid and gaseous CO₂ with a second compressed gas having a lower heat of compression than CO₂, leaving a majority of the secondary gas in the vapor phase and serving to prevent thermal shock upon subsequent venting thereof.
 37. The method according to claim 20, wherein said drying composition is a supercritical drying composition, wherein said immersing and removing steps are carried out in an enclosed chamber, and wherein said homogenizing step is carried out by adding a second material to said supercritical drying composition so that it is converted to a liquid drying composition.
 38. The method according to claim 20, wherein said homogenizing step is carried out by diluting said drying composition with additional carbon dioxide.
 39. A method according to claim 20, wherein said drying step is initiated with said drying composition in a liquid state, and after a period of time said homogenizing step is carried out by diluting said drying composition with pure liquid CO₂, then heating said drying composition to produce a supercritical fluid, after which said removing step is carried out while maintaining the temperature of the fluid and gas above the critical temperature of CO₂.
 40. A method according to claim 20, where said drying step is preceded by a development step that incorporates aqueous developers and pure water in the same chamber as in the drying step. 